The present invention relates to a linear compressor which compresses and externally supplies gas by driving a piston fit within a cylinder to move back and forth by a linear motor.
In recent years, there have been developed linear compressors as a mechanism for compressing and supplying refrigerant gas in a refrigeration system. As shown in FIG. 26, for example, a linear compressor includes a is cylindrical housing 101 having a bottom, a magnetic frame 102 of a low carbon steel formed at the upper end opening of housing 101, a cylinder 103 formed in the central portion of magnetic frame 102, a piston 105 fit within cylinder 103, capable of moving back and forth and defining a compression chamber 104 in the space of cylinder 103, and a linear motor 106 serving as a driving source to drive piston 105 to reciprocate.
Linear motor 106 has an annular permanent magnet 107 provided at an outer concentric position with cylinder 103 and fixed to housing 101. A magnetic circuit formed of magnet 107 and magnetic frame 102 produces a magnetic field B in a cylindrical gap 108 concentric with the center of cylinder 103. A cylindrical mobile body 109 having a bottom, formed of resin and integrally fixed to piston 105 is provided in gap 108 in the center, and a coil spring 110 for elastically supporting mobile body 109 and piston 105 and permitting them to reciprocate is fixed to housing 101.
An electromagnetic coil 110 is wound around the outer circumference of mobile body 109 at a position opposite to magnet 107, ac current at a prescribed frequency is passed through a lead (not shown) to drive coil 111 and mobile body 109 by the function of a magnetic field through gap 108 to force piston 105 to move back and forth within cylinder 103, and gas pressure is generated at a prescribed cycle in compression chamber 104.
Meanwhile, as shown in FIG. 27, there is known, as a representative refrigerating system, a closed a type refrigerating system in which a linear compressor 121 (compressor), a condenser 122, an expansion valve 123 and an evaporator 124 are connected by a gas flow path pipe 125. Linear compressor 121 is used as a device to compress to a high pressure a refrigerant gas evaporated at evaporator 124 and taken in through gas flow path pipe 125, and let out thus pressurized refrigerant gas to condenser 122 through gas flow path pipe 125.
Therefore, as shown in FIG. 26, compression chamber 104 is connected with gas flow path pipe 125 outside housing 101 through a valve mechanism 112 provided at the upper end portion of cylinder 103. Valve mechanism 112 includes an inlet valve 112a which permits only refrigerant gas from evaporator 124 to enter through gas flow path pipe 125, and an outlet valve 112b which permits only refrigerant gas to be let out to condenser 122 through gas flow path pipe 125. Inlet valve 112a allows gas to flow toward compression chamber 104 by the difference in pressure of refrigerant gas between gas flow path pipe 125 on the low pressure side and compression chamber 104.
Outlet valve 112b allows gas to flow toward gas flow path pipe 125 on the high pressure side by the difference in pressure of refrigerant gas between compression chamber 104 and gas flow path pipe 125 on the high pressure side. Note that inlet valve 112a and outlet valve 112b are both energized by a plate spring.
Thus, in the conventional device, refrigerant gas taken in from inlet valve 112a is compressed to a high pressure in compression chamber 104, and supplied to condenser 122 through outlet valve 112b. 
In addition, in recent years, as disclosed by Japanese Patent Laying-Open No. 2-154950, for example, there has been proposed a technique of improving the efficiency by providing compression chambers on both sides in a housing and alternately operating two pistons by a single linear motor.
The linear compressors are divided into two kinds, in other words, those like a coil mobile linear compressor as disclosed by Japanese Patent Application No. 8-179492, and those like a magnet mobile type linear compressor as disclosed by Japanese Patent Application No. 8-108908. These two kinds of linear compressors both produce compressed gas in a compression chamber by driving a piston to move back and forth using a driving force obtained from a linear motor.
The above-described linear compressors are, however, encountered with various problems as follows.
First Problem
The conventional single piston type linear compressor is largely affected by non-linear force produced within a compression chamber associated with in taking/compression/exhaustion of a gas, and the thrust of the motor cannot be linearized, which makes it difficult to improve the efficiency.
Furthermore, the neutral point of the piston fluctuates with the fluctuation of load at the time of activation for example, and the stroke of the piston cannot be readily controlled.
Second Problem
In conventional linear compressor 121, piston 105 is driven by linear motor 106 to move up and down within cylinder 103, and mobile body 109 also moves up and down, which causes gas present in the space in the magnetic circuit formed by magnetic frame 102, permanent magnet 107 and mobile body 109 and gas present in the space inside the mobile body on the back side of piston 105 surrounded by the inner surface portion of mobile body 109 perform compression/expansion work as mobile body 109 moves up and down, which could lead to irreversible compression losses in linear compressor 121.
As a countermeasure, gap 108 may be sufficiently secured to provide a sufficient gap between magnetic frame 102 and mobile body 109 and between permanent magnet 107 and electromagnetic coil 111, but the thrust of linear motor 106 decreases in this case, which lowers the operation efficiency of linear compressor 121.
Third Problem
In linear compressor 121 as described above, piston 105 is driven by linear motor 106 to move up and down within and slidably in contact with cylinder 103, and a kind of slide bearing is formed between the piston and the cylinder.
In the conventional structure as described above, however, a force (radial force) in the direction vertical to the moving direction of the piston is generated because of the problem of processing precision and a distortion in the electromagnetic force of the electromagnetic coil, and if the radial force is large, the operation efficiency may be lowered because of frictional losses, the life of the device may be shortened because of abrasion at a gas seal portion provided at piston 105, and the refrigerant may be contaminated by dust created by abrasion.
Fourth Problem
The linear compressor disclosed by Japanese Patent Laying-Open No. 2-154950 employs a magnet mobile type linear motor driving method rather than the coil mobile type as described above and shown in FIG. 26, force by magnetic field in the direction vertical to the moving direction of the piston is applied to the piston, the piston portion is prone to abrasion and therefore the compressor is not suitable for such use.
Therefore, in a linear compressor to be used for a long period of time, the driving method of the linear motor may be changed to the coil mobile type according to which force by the magnetic field of the linear motor acts only in the same direction as the mobile direction of the piston.
Furthermore, gas present in the back space of the piston performs compression/expansion work as the piston moves back and forth, which could lead to irreversible compression losses in linear compressor 121.
In addition, in the conventional linear compressor, the central position of the stroke of piston cannot be controlled at a prescribed position, and therefore highly efficient operation cannot be performed.
Fifth Problem
In the refrigerating system as described above, compressed gas obtained in the compression chamber of the linear compressor is supplied to condenser 122 from outlet valve 112b through gas flow path pipe 125, vibration noise in the pipe caused by the pulsation of the gas or valve operation noise are generated at the time of opening/closing outlet valve 112b, and therefore there should be provided an outlet muffler 131 for controlling noise in the pipe on the downstream side of outlet valve 112b. 
The above-described 2-piston type linear compressor must be provided with two such outlet mufflers for noise control, and two outlet pipes must be coupled preceding to condenser 122, which could increase the size of the entire device.
Sixth Problem
In the refrigerating system as described above, the piston is permitted to move back and forth in the cylinder, and a coil spring is often used as a member for elastically supporting the piston to the housing. In recent years, a plate shaped piston spring has been proposed which is advantageous over a conventional coil spring in terms of durability and positional restriction in the mobile direction, and various attempts have been made for improvements thereof (T. Haruyama, et al.: Cryogenic Engineering 1992 fall lecture meeting B2-4, p166).
The plate shaped piston spring is generally called xe2x80x9csuspension springxe2x80x9d, and has a disk shaped plate spring 920a having a plurality of spiral cut out portions 920b equidistantly provided toward the central portion as shown in FIG. 28.
Using plate shaped suspension spring 120 as the piston spring, the stroke central position of the piston can be fixed by a simple device.
Plate shaped suspension spring 920, however, cannot restrict the deviation of the axis of the piston in the vicinity of upper and lower supporting points of the piston where the spring is fully extended. As a result, the piston may locally abut against the cylinder for some reasons and abrasion may be caused at the piston portion.
Seventh Problem
Meanwhile, the magnet mobile type linear compressor as disclosed by Japanese Patent Application No. 8-108908 may be advantageously formed into a compact shape, but since attracting force by magnetic force is used as the driving force of the linear motor to force the piston to move up and down, force in the direction vertical to the upward and downward movement of the piston is likely to be generated. The driving force is lost because of friction between the piston and the cylinder and friction at the bearing portion of the shaft supporting the piston, which lowers the efficiency. Therefore, an expensive gas bearing or the like should be used for the bearing portion of the shaft supporting the piston.
The coil mobile type linear compressor as disclosed by Japanese Patent Application No. 8-179492 on the other hand employs Lorentz force as the driving force of the linear motor, and therefore the deviation of the axis is less likely as compared to the magnet mobile type linear compressor. In order to obtain the same output as by the magnet mobile type linear compressor, however, the device is generally increased in size.
It is therefore a first object of the invention to provide a highly efficient linear compressor which permits the stroke of a piston to be readily controlled.
Then, a second object of the invention is to provide a linear compressor whose efficiency is improved by reducing a gap in a magnetic circuit during the reciprocating movement of a mobile body as much as possible and preventing an irreversible compression loss.
Then, a third object of the invention is to provide a linear compressor whose efficiency is improved and whose life is prolonged.
Then, a fourth object of the invention is to provide a linear compressor having compression chambers on both sides in a housing, and compressing and externally supplying gas by driving a coil mobile type linear motor, wherein an irreversible compression loss is prevented in the back space of the piston by a simple structure, and the stroke central position of the piston is fixed.
Then, a fifth object of the invention is to provide a linear compressor having compression chambers on both sides in a housing, and compressing and externally supplying gas by driving a coil mobile type linear motor, wherein the stroke central position of the piston is fixed by a simple structure, abrasion at the piston portion is prevented by restricting the deviation of the axis of the piston when the piston is driven to reciprocate, and the life of the device is prolonged.
A sixth object of the invention is to provide a linear compressor which permits prevention of loss in the driving force, caused by friction between a piston and a cylinder and friction at the bearing portion of a shaft supporting the piston and the size of the device to be reduced.
A linear compressor according to a first aspect of the invention for generating a compressed gas includes two pairs of pistons and cylinders provided coaxially and facing opposite to each other, a shaft provided with a piston at each of its both ends, an elastic member coupled to the shaft for returning the piston departed from the neutral point to the neutral point, and a linear motor for forcing the shaft to axially move back and forth to generate a compressed gas alternately by the two pairs of pistons and cylinders.
Thus, the non-linear force of the compressed gas acting upon the pistons can be divided into two/reversed in phase. As a result, as compared to a conventional structure provided only with a single piston, the motor thrust may be reduced and linearized, which improves the efficiency. Furthermore, the size of the device may be reduced, and vibration/noises may be reduced as well. In addition, the position of the neutral point of the piston does not fluctuate if the load fluctuates, the stroke of the piston may be readily controlled simply by controlling the driving current of the linear motor.
Furthermore, more specifically, a vibrating portion including the two pistons, the shaft and the elastic member has a predetermined resonant frequency, and the linear motor forces the shaft to reciprocate at the resonant frequency.
Thus, the shaft may be reciprocated at the resonant frequency of the vibrating portion, which further improves the efficiency.
In addition, more specifically, the regaining force of the elastic member to return the piston departed from the neutral point to the neutral point is set larger than the force of the compressed gas acting upon the piston.
Thus, the non-linear force of the compressed gas acting upon the piston may be restricted to a small level, which further improves the linearity of the motor thrust.
A linear compressor according to a second aspect of the invention includes a cylinder provided within a housing, a piston fit within the cylinder, capable of moving back and forth and defining a compression chamber within the cylinder, a linear motor having a cylindrical mobile body with a bottom fixed integrally to the piston at the central portion and provided in a gap formed in part of a magnetic circuit of a magnet and a magnetic frame for driving the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body. The linear compressor externally supplies gas compressed within the compression chamber and has a gas leaking device provided at the mobile body and/or the magnetic frame.
Thus providing the gas leaking device at the mobile body and/or magnetic frame may prevent an irreversible compression loss associated with the reciprocating movement of the mobile body.
More specifically, the structure of the gas leaking device includes a first leak hole provided at the magnetic frame for leaking gas, a buffer space portion communicated with the first leak hole, and a second leak hole provided at the mobile body for leaking gas.
The use of the structure prevents compression/expansion work of gas in the space portion of the magnetic circuit formed by the magnetic frame, permanent magnet and mobile body and in the inner space portion of the mobile body surrounded by the rear side of the piston and the inner portion of the mobile body.
Furthermore, the compressor according to this aspect further includes a piston shaft provided between the piston and the mobile body, a spring receiving portion provided at the cylinder on the rear surface of the piston and having the piston shaft fit being capable of moving back and forth therein, a first coil spring fit into the piston shaft and provided between the spring receiving portion and the mobile body, a second coil spring provided between the bottom surface of the housing and the mobile body, and a third leak hole for leaking gas to communicate the rear surface space portion of the piston and the mobile body inner space portion having the first coil spring wound therearound.
Use of the structure wherein the first and second coil springs are provided on both sides through the mobile body permits the stroke central position of the piston to be readily stably controlled in a fixed manner, and permits the spring constant to be set larger than the conventional cases within the same device dimension. In addition, gas compression/expansion work may be prevented in the piston rear surface space in association with the upward and downward movement of the piston.
A linear compressor according to a third aspect of the invention includes a cylinder provided within a housing, a piston fit within the cylinder with a fine gap, capable of moving back and forth and defining a compression chamber within the cylinder, a piston shaft having one end portion fixed to the piston, a linear motor in which a cylinder mobile body with a bottom integrally fixed to the piston shaft is provided at a gap formed at a part of a magnetic circuit formed of a magnet and a magnetic frame and which drives the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body, and a rolling bearing at the inner circumference, and there is provided a guide portion for slidably retaining the piston shaft at the rolling bearing.
By using the structure, the piston shaft is directly supported by the rolling bearing so that the direction of the linear movement of,the piston is defined, and therefore, clearance seal may be achieved between the piston and cylinder.
More specifically, the fine gap as described above is within the range in which a gas seal is formed to the cylinder in association with the reciprocating movement of the piston, and is preferably set not more than 5 xcexcm.
The guide portion is formed of a first guide portion provided at the cylinder on the rear side of the piston and a second guide portion provided at the bottom surface of the housing and includes a first coil spring provided between the first guide portion and the mobile body and a second coil spring provided between the second guide portion and the mobile body.
Use of the structure permits the stroke central position of the piston to be controlled readily stably and permits the spring constant within the same device dimension to be set larger than the conventional cases.
A linear compressor according to a fourth aspect of the invention includes a cylinder provided within a housing, a piston fit within the cylinder, capable of moving back and forth, and defining a compression chamber within the cylinder, a piston shaft having one end portion fixed to the piston, and a linear motor in which a cylindrical mobile body having a bottom integrally fixed to the piston shaft is provided in a gap formed at a part of a magnetic circuit formed of a magnet and a magnetic frame and which drives the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body. The linear compressor externally supplies gas compressed within the compression chamber and is provided with a rolling bearing at the cylinder or the piston, through which the piston is moved back and forth along the cylinder.
Use of this structure permits the piston to slide along the cylinder through the rolling bearing, there is no necessity to provide a gas seal member at the piston, and therefore degradation in the operation efficiency by friction loss between the piston and the cylinder as the piston moves back and forth may be prevented.
More specifically, the structure includes a spring receiving portion provided at the cylinder on the rear surface of the piston, to which the piston shaft is freely fit and capable of moving back and forth, a first coil spring provided between the spring receiving portion and the mobile body, and a second coil spring provided between the bottom surface of the housing and the mobile body.
Use of this structure permits the stroke central position of the piston to be controlled readily stably, and permits the spring constant within the same device dimension to be set larger than the conventional cases.
Now, a linear compressor according to a fifth aspect of the invention for compressing gas within a compression chamber and externally supplying the compressed gas includes first and second cylinders provided on both sides within a housing, first and second pistons fit, capable of moving back and forth within the first and second cylinders and defining compression chambers within the first and second cylinders, respectively, a piston shaft having end portions fixed to the first and second pistons, a linear motor in which a cylindrical mobile body with a bottom integrally fixed to the piston shaft is provided in a gap formed at a part of a magnetic circuit formed of a magnet and a magnetic frame and which drives the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body, coil springs provided having the mobile body therebetween for elastically supporting the first and second pistons so that they can move back and forth within the first and second cylinders, respectively, the insides of the first piston, piston shaft and second piston are hollow and communicated with each other, and the rear surface space of the first piston and the rear surface space of the second piston are communicated with each other.
Use of this structure permits gas in the rear surface portion to be communicated through the first piston, piston shaft and second piston in association with the reciprocating movement of the first and second pistons, no compression/expansion work is performed and therefore no irreversible compression loss is caused. In addition, in the linear compressor having compression chambers on both sides of the housing, by providing coil springs on both sides through the mobile body, the stroke central positions of the first and second pistons may be readily controlled stably, so that a prescribed spring constant may be established.
Furthermore, the rear surface space of the first piston and the rear surface space of the second piston are communicated by providing a first leak hole at the first piston to communicate the rear surface space of the first piston and the hollow inside of the first piston as well as by providing a second leak hole at the second piston to communicate the rear surface space of the second piston and the hollow inside of the second piston.
Use of this structure may prevent irreversible compression loss with the simple structure.
Now, a linear compressor according to a sixth aspect of the invention includes first and second cylinders provided within a housing on both sides, first and second pistons fit within the first and second cylinders, capable of moving back and forth and defining compression chambers within the first and second cylinders, respectively, a piston shaft having end portions fixed to the first and second pistons, a linear motor in which a cylindrical mobile body having a bottom integrally fixed to the piston shaft is provided in a gap formed at a part of a magnetic circuit formed of a magnet and a magnetic frame and which drives the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body, and coil springs provided having the mobile body therebetween for elastically supporting the first and second pistons within the first and second cylinders, respectively so that they can move back and forth, the first piston, piston shaft and second piston are made hollow inside and communicated with each other, compressed gas from the compression chamber within the first cylinder is supplied externally through the hollow portions of the first piston and piston shaft, while compressed gas from the compression chamber within the second cylinder is externally supplied through the hollow portions of the second piston and piston shaft.
Use of this structure permits the coil springs to be provided on both sides through the mobile body, the stroke central positions of the first and second pistons to be more easily stably controlled, and therefore a prescribed spring constant may be established.
Noises such as vibrating sound due to gas pulsation generated at the time of letting out compressed gas may be shielded within the housing, and therefore there is no necessity to additionally provide an outlet muffler for preventing the noises.
More specifically, first and second outlet valves for letting out compressed gas onto the hollow portions of the first and second pistons are provided at the first and second pistons, and compressed gas from the compression chambers are externally supplied through the hollow portions of the first and second pistons, the hollow portion of the piston shaft, the hollow mobile space portion formed within the mobile body and a communication tube capable of extending/contracting which is provided between an end side of the mobile body space portion and the main body housing. The communication tube is formed of a bellows type tube or a coil type tube.
Use of this structure permits noises to be shielded within the housing by a simple structure and the entire device to be made more compact.
Now, a linear compressor according to a seventh aspect of the invention includes first and second cylinders provided at both sides within a housing, first and second pistons fit within the first and second cylinders, capable of moving back and forth therewithin and defining compression chambers within the first and second cylinders, respectively, a piston shaft having end portions fixed to the first and second pistons, a linear motor in which a cylindrical mobile body having a bottom integrally fixed at the piston shaft is provided in a gap formed at a part of a magnetic circuit formed of a magnet and a magnetic frame and which drives the piston to move back and forth by supplying ac current at a prescribed frequency to an electromagnetic coil wound around the outer circumference of the mobile body, plate shaped piston springs provided between the housing and the piston shaft for elastically supporting the first and second pistons within the first and second cylinders, respectively so that they can move back and forth therewithin, and a gas bearing portion to let a part of compressed gas from the compression chambers within the first and second cylinders to be ejected to restrict the positions of the first and second pistons in the axial directions.
By using this structure, as the first and second pistons are positioned near the neutral points, the axial positions of the first and second pistons are restricted by the plate shaped piston springs, while as the first and second pistons are positioned near the upper and lower supporting points, the axial positions of the first and second pistons are restricted by the gas bearing portion. Therefore, the stroke central positions of the first and second piston may be controlled stably by a simple structure, abrasion at the piston portion may be prevented by limiting the deviation of the axes of the pistons when the first and second pistons are driven to move back and forth, so that the life of the device may be prolonged.
More specifically, there are provided a first communication path for supplying compressed gas from the compression chamber in the first cylinder to the gas bearing portion, and a second communication path for supplying compressed gas from the compression chamber within the second cylinder to the gas bearing portion.
Use of this structure permits gas to be supplied to the gas bearing portion using a part of compressed gas from the compression chamber, therefore there is no necessary for providing additional means for supplying gas, and the entire device may be made more compact.
More preferably, the first communication path is formed in the first piston and piston shaft, and the second communication path is formed in the second piston and piston shaft.
Use of this structure permits gas to be blown toward the side of the bearing from the piston shaft side, and therefore the entire structure may be more simplified than otherwise.
The gas bearing portion may be formed of a first gas bearing portion provided at the first cylinder on the rear side of the first piston for restricting the axial position of the first piston and a second gas bearing portion provided at the second cylinder on the rear side of the second piston for restricting the axial position of the second piston.
By using this structure, the first gas bearing limits the deviation of the axis when the first piston is positioned near the upper and lower supporting points, while the second gas bearing portion limits the deviation of the axis when the second piston is positioned near the upper and lower supporting points.
Furthermore, the first and second pistons may be freely fit capable of moving back and forth with a fine gap left within the first and second cylinders, more specifically, a fine gap set to be not more than 10 xcexcm.
By using this structure, gas seal is formed between the cylinders and the pistons in association with the reciprocating movement of the pistons, and it is not necessary to additionally provide a gas shield member at the circumferential side surface of the pistons.
As a result, clearance seal without local bias may be implemented between the pistons and the cylinders, and degradation in the operation efficiency due to friction loss between the pistons and the cylinders as the pistons move back and forth may be prevented.
A linear compressor according to an eighth aspect of the invention includes a shaft having a piston, a cylinder having a compression chamber to accommodate the piston, a casing provided integrally with the cylinder for accommodating the shaft, a linear motor coupled with the shaft and the casing for providing the piston with reciprocating movement in order to generate the compressed gas in the compression chamber, a first elastic member coupled with the shaft for returning the piston departed from the neutral point to the neutral point, a second elastic member coupled to the shaft for preventing the deviation of the axis of the shaft.
More preferably, a vibrating portion including the piston, shaft, first elastic member, second elastic member and compressed gas has a prescribed resonant frequency, and the linear motor drives the shaft to move back and forth at the resonant frequency.
More preferably, the linear motor includes a coil provided on the casing, and a permanent magnet provided on the shaft and the first elastic member is provided to be accommodated within an inner space provided at the permanent magnet.
More preferably, the first elastic member is a coil spring, and the second elastic member is a suspension spring.
As in the foregoing, in the linear compressor according to the eighth aspect, the first elastic member for returning the piston to the neutral point, and the second elastic member for preventing the deviation of the axis of the shaft are used.
As a result, in an application to a magnet mobile type linear compressor, for example, the deviation of the axis of the piston is prevented by the second elastic member, and compression of refrigerant gas may be efficiently performed.
Furthermore, in an application to a magnet mobile type linear compressor, by accommodating the first elastic member within the inner space provided at the permanent magnet provided at the shaft, the inner space within the linear compressor may be efficiently used, so that the linear compressor may be made more compact.